Fix #18658: Handle varargs of generic types in JSExportsGen.

When extracting the type of a varargs parameter, we have to go back
to before erasure. However, that gives us a non-erased type inside
as well. We need to re-erase that type to get something sensible
for the back-end.

compiler/src/dotty/tools/dotc/transform/sjs/JSSymUtils.scala

@@ -185,7 +185,7 @@ object JSSymUtils {
         val list =
           for ((name, info) <- paramNamesAndTypes) yield {
             val v =
-              if (info.isRepeatedParam) Some(info.repeatedToSingle.widenDealias)
+              if (info.isRepeatedParam) Some(TypeErasure.erasure(info.repeatedToSingle))
               else None
             name -> v
           }

tests/sjs-junit/test/org/scalajs/testsuite/jsinterop/NonNativeJSTypeTestScala3.scala

@@ -0,0 +1,21 @@
+package org.scalajs.testsuite.jsinterop
+
+import org.junit.Assert.*
+import org.junit.Test
+
+import scala.scalajs.js
+import scala.scalajs.js.annotation.*
+
+class NonNativeJSTypeTestScala3 {
+  @Test
+  def overloadWithVarargOfGenericType(): Unit = {
+    class OverloadWithVarargOfGenericType extends js.Object {
+      def overloaded(x: Int): Int = x
+      def overloaded(xs: (Int, Int)*): Int = xs.size
+    }
+
+    val obj = new OverloadWithVarargOfGenericType
+    assertEquals(5, obj.overloaded(5))
+    assertEquals(2, obj.overloaded((1, 2), (3, 4)))
+  }
+}